1. Field of the Invention
The invention relates generally to the field of electronic circuits and more particularly to systems and methods for reducing or eliminating the variability of timing associated with history effects in silicon-on-insulator (SOI) devices.
2. Related Art
Sense amplifiers are devices that are commonly used to amplify small voltage differences. For instance, a voltage difference on the order of 10–100 millivolts may be amplified to a voltage of 1–3 volts. In one type of sense amplifier, voltages are detected on a pair of input lines, and corresponding amplified voltages are generated on a pair of output lines. In another type of sense amplifier, voltages are detected on a pair of data lines, and these same data lines are then pulled to levels that effectively amplify the original voltage difference. That is, the data line that initially has the higher voltage is pulled to a predetermined high voltage (e.g., Vdd), and the data line that initially has the lower voltage is pulled to a predetermined low voltage (e.g., ground).
Referring to FIG. 1, a diagram illustrating the design of an exemplary latch-type sense amplifier in accordance with the prior art is shown. Sense amplifier 100 consists of seven interconnected transistors. Data lines 111 and 112 are coupled to nodes 131 and 132 through PMOS transistors 121 and 122, respectively. The gates of transistors 121 and 122 are both coupled to signal line 132 to receive a transfer signal, /xfer. Intermediate node 131 is coupled to a power supply voltage, Vdd, through transistor 141, and is coupled to ground through transistors 151 and 160. Similarly, intermediate node 132 is coupled to the power supply voltage through transistor 142, and is coupled to ground through transistors 152 and 160. When transistors 141 and 142 are switched on, they pull the voltages at nodes 131 and 132, respectively, toward Vdd, and when transistors 151 and 152 are switched on, they pull the voltages at nodes 131 and 132, respectively, toward ground. The gates of transistors 141 and 151 are tied together, and are cross coupled to node 132. The gates of transistors 142 and 152 are likewise tied together, and are cross coupled to node 131.
The operation of sense amplifier 100 will be described with reference to FIG. 2. FIG. 2 is a diagram illustrating the voltage levels at nodes 131 and 132, as well as an enable signal, SAEN. Initially, both node 131 and node 132 are pre-charged to Vdd. At some time, t1, the voltage at either node 131 or node 132 begins to drop below Vdd. Sense amplifier 100 is enabled, or “fired,” when a voltage difference has developed between nodes 131 and 132. In this embodiment, sense amplifier is enabled when the voltage difference is about 100 millivolts. FIG. 2 shows the enable signal, SAEN, going high at time t2, thereby enabling the sense amplifier.
It should be noted that “enabled,” as used here, refers to the coupling of the upper portion of the circuit to ground through transistor 160 (i.e., by switching the transistor on.) Likewise, “disabled,” or “not enabled” are intended to refer to the decoupling of the upper portion of the circuit from ground by switching off transistor 160. The use of this terminology is not intended to imply that, when the sense amplifier is “disabled,” the entirety of the circuit is disabled, and it is clear that there are various processes that occur within sense amplifier 100 (e.g., precharging the intermediate nodes, developing voltage differences between the intermediate nodes, etc.) while the sense amplifier is “disenabled” (i.e., when transistor 160 is switched off.) These terms will be used in the same manner below with respect to the descriptions of the various embodiments of the invention.
As pointed out above, nodes 131 and 132 are cross coupled to the gates of the transistors on the opposite sides of sense amplifier 100. Because nodes 131 and 132 are both initially pre-charged to Vdd, transistors 141 and 142 are initially switched off, while transistors 151 and 152 are initially switched on. Even though transistors 151 and 152 are switched on, however, they do not pull the voltages at nodes 131 and 132 to ground because enable signal SAEN is low, so transistor 160 is switched off. As the voltage at one of nodes 131 or 132 begins to drop below Vdd, the transistors on the opposite side of sense amplifier 100 are switched on and off less strongly. For example, as shown in FIG. 2, the voltage at node 132 begins to drop below Vdd, so transistor 141 is switched off less strongly than at time to, and transistor 151 is switched on less strongly than at time t0. As a result, the voltage at node 131 is more strongly pulled toward Vdd than the voltage at node 132.
Sense amplifier 100 is enabled at time t2 when SAEN goes high. More specifically, transistor 160 is switched on, so that nodes 131 and 132 are pulled toward ground through transistors 151 and 152, respectively. Because the voltage at node 131 is higher than the voltage at node 132, transistor 152 is switched on more strongly than transistor 151. As a result, the voltage at node 132 (which had already dropped below the voltage at node 131) is pulled more strongly to ground than the voltage at node 131. Thus, the voltage at node 131 is pulled to Vdd, while the voltage at node 132 is pulled to ground. (See FIG. 2 at time t3.)
Sense amplifier 100, as well as the circuitry that generates enable signal SAEN, may be manufactured using silicon-on-insulator (SOI) technology. Transistors that are manufactured using SOI technology comprise layers of silicon and are deposited on an insulating substrate. Because these silicon layers are built on top of an insulator, the bodies of the transistors typically “float,” so that the body voltages of the transistors may have different values at different times. For example, if a transistor is between two nodes that are both at ground, the body voltage of the transistor will be close to ground as well. If, the other hand, one of these nodes is at Vdd, the body voltage of the transistor will be higher than ground (e.g., 100 millivolts.) Thus, the body voltage of the transistor will vary with the state of the circuit in which it is used (i.e., the voltages at the nodes.)
Variations in the body voltages of the transistors are important because the threshold voltage of each transistor varies with the body voltage of the transistor. This, in turn, is important because the response of the transistor (e.g., the delay with which the transistor switches) varies with the threshold voltage of the transistor. The transistor response is therefore dependent, to some extent, upon the previous state of the circuit in which it is used. This dependence upon the previous state is sometimes referred to as the history effect.
The history effect is important in the context of sense amplifier 100 because it may cause variations in the timing of enable signal SAEN. In FIG. 2, SAEN goes high at time t2. At time t2, the voltage difference between nodes 131 and 132 is anticipated to be great enough that the lower voltage (in this particular example, the voltage at node 132) is pulled to ground, while the higher voltage (in this example, the voltage at node 131) is pulled to Vdd. If, however, the history effect causes variations in the timing of signal SAEN, this signal may go high either sooner or later than anticipated. If SAEN goes high too early, there may not have been sufficient time for the voltage difference between nodes 131 and 132 to develop. If the voltage difference is too small, the nodes may be pulled to the wrong voltages (i.e., the voltage at node 132 may be pulled high, while the voltage at node 131 may be pulled low.) In this case, the sense amplifier has malfunctioned and inverted the signals that were intended to be generated on the data lines.
The history effect may alternatively cause SAEN to be delayed. In this case, there will have been sufficient time for the voltage difference to develop between nodes 131 and 132, so sense amplifier 100 will operate properly, in terms of amplifying the signals without converting them. It is necessary, however, to allow sufficient time after SAEN goes high for the data lines to be pulled to ground and to Vdd. In other words, sense amplifier 100 operates properly, but it operates more slowly than if the timing is as anticipated. It therefore be necessary to operate sense amplifier 100 at a slower speed than desired.